In genetic and cytological studies of the haploid fungus Neurospora, chromosome rearrangements are being analyzed and used for a variety of problems. Several types (e.g., insertional translocations) recurrently generate via meiosis a class of viable progeny containing nontandem duplications. Their properties are of interest for comparison with similar rearrangements in diploid organisms, including man, where direct study is difficult. Duplication-generating aberrations are being used for studying vegetative incompatibility, regulation, complementation and dominance, mapping by duplication coverage, and the derivation of new sequences. Chromosome structure and behavior in meiosis and mitosis are being studied cytologically and cytogenetically, using chromosome rearrangements and meiotic mutants. Of special interest are mutants possessing the syndrome radiation sensitivity, meiotic impairment, increased instability of duplications, and sensitivity to histidine and certain other amino acids. With many rearrangments, deletion of duplications occurs nonrandomly, so as to restore normal chromosome sequence. A recombination model is being examined which would explain this lack of parity. Chiasma interference is being investigated in light of cytological information and of genetic data on the crossing over of flanking markers that accompanies intragenic recombination. Wild-collected strains are being used as a source of genetic variants and information on variation in nature. Spore killer genes are being analyzed, which show meiotic drive and result in death of meiotic products that contain the normal, sensitive allele. Information is being assembled on the phenotypes, characteristics and map locations of all chromosomal loci of Neurospora.